In the production of minerals, e.g., oil and gas, certain reservoir properties of a subterranean reservoir must be determined. Two of the most important of these properties are the porosity and permeability of the reservoir. Porosity of a material is the ratio of the aggregate volume of its void or pore spaces (i.e., pore volume) to its gross bulk volume and, in the case of an oil or gas reservoir, is a measure of the capacity within the reservoir rock which is available for storing oil or gas. Permeability of a material is a measure of the ability of the material to transmit fluids through its pore spaces and is inversely proportional to the flow resistance offered by the material. Another important parameter is pore compressibility, which is change in porosity, or pore and bulk volume, as a function of pressure.
Normally, these parameters are determined by taking core samples from the reservoir and carrying out well-defined measurement techniques on the samples. There are several techniques available for making such measurements, many of which are described in PETROLEUM PRODUCTION ENGINEERING - DEVELOPMENT by L. C. Uren, Fourth Edition, McGraw-Hill Book Company, Inc., 1956, pps. 660-669. Another standard reference is American Petroleum Institute, API Recommended Practice for Core-Analysis Procedure, API RP 40, 1960, 55 pp.
A more recently applied technique involved computed tomography (CT) technology which has been in use in the medical field for a number of years. CT scanning instruments produce a cross-sectional view through the subject material along any chosen axis. The advantages of CT scanning over conventional radiography is found in its ability to display the electron density variations within the object scanned in a two-dimensional X-ray image. In medical CT scanners, an X-ray source and a detector array circle a patient in a period of about 2 to 9 seconds and produces an image with maximum resolutions of 0.25 mm in the X-Y plane. This plane can be moved in discrete intervals to obtain information in three dimensions. For more details of such medical CT scanners, reference may be made to U.S. Pat. No. 4,157,472 to Beck, Jr. and Barrett (Assignee: General Electric Company) and U.S. Pat. No. 4,399,509 to Hounsfield (Assignee: EMI Limited).
Many other applications of CT scanning can also be made. For example, in an article entitled, "Computed Tomographic Analysis of Meteorite Inclusions", Science, pps. 383-384, Jan. 28, 1983, there is described the non-destructing testing of meteorites for isotopic anomalies in calcium- and aluminum-rich inclusions of heterogeneous materials, such as Allende. The CT scanning equipment described in such article is the Deltascan 2020 from Technicare. In a further application, CT scanning has been applied to the non-destructive testing of wood materials, such as for disease in living trees, see U.S. Pat. No. 4,283,629 to Habermehl. In a yet further application, CT scanning has been applied to the examination of non-living objects, such as motors, ingots, pipes, etc., see U.S. Pat. No. 4,422,177 to Mastronardi, et al. (Assignee: American Science and Engineering, Inc.).
More recently, the CT scanning and technology has been applied to the field of energy research for examining the interior of stationary or slowly changing earth materials, such as coal, shale and drilling cores. Processes involved in coal gasification and combustion have been monitored using time-lapse CT imagery to observe changes in density (e.g., thermal expansion, fracturing, emission of gases, consumption by combustion) during progressive heating in a controlled atmosphere. Core flooding experiments can now be carried out with CT scanning to aid in enhanced oil recovery and fluid mobility control. For example, the permeability of materials within core samples to various fluids at varying conditions of temperature and pressure can be determined. Such experiments might involve flushing a fluid through a core sample and monitoring the shape of the fluid fronts. By subtracting the images of the cores before and after flooding, the exact shapes of the fluid front was determined. Such core flood experiments allows the interior of the core sample to be observed without disturbing the sample. The sweep efficiency and flow paths of fluids of interest may now be studied on the scale of millimeters. The penetration of X-rays allows experiments to be performed with up to 4-inch diameter core samples.
Drilling fluids can be analyzed by CT scanning as such fluids are characterized by high-density brines, various organics and several compositionally different weighting agents. Formation damage can be investigated since CT scanning can detect migration of clays, absorption of organics and the reversibility of completion fluid penetration. Shale oil recovery can be aided as CT scanning could detect penetration by solvents and could directly measure structure changes on retorting.
U.S. Pat. No. 4,649,483 to Dixon discloses a method for determining fluid saturation in a porous media through the use of CT scanning. Multi-phase fluid saturation in a sample of a porous media is determined through computer tomographic scanning. The sample is scanned with X-rays of differing energies in both the fluid saturated and fluid-extracted states. Each of the extracted fluids is also scanned at differing X-ray energies. The computed tomographic images produced are utilized in the determination of the X-ray mass attenuation coefficients for the sample and the extracted fluids. From these mass attenuation coefficients, the weight fractions and volume fractions of each of the extracted fluids are determined.
U.S. Pat. No. 4,688,238 issued Aug. 18, 1987, to Sprunt et al. discloses a method for using CT scanning over a range of confining pressures on a core sample to determine pore volume change, pore compressibility and core fracturing. A core sample with a surrounding elastic jacket is placed in a confining pressure cell. Pressure is applied to the cell to press the jacket into contact with the surface of the sample. The pressure in the cell is increased stepwise over a plurality of pressure points. The sample is scanned at a plurality of locations with X-rays at each of the pressure points. Computed tomographic images of the sample are produced for each of the X-ray scans. The conformance of the jacket to the sample is determined from these computed tomographic images. From such conformance, a range of confining pressures is determined over which pore volume and pore compressibility of the sample are measured without being affected by improper conformance of the jacket to the surface of the sample. Also rock fracturing is determined from the pressure at which crushing of the sample destroys permeable channels within the sample and results in a permeability measurement that is lower than the actual permeability of the sample.